Table of contents

1. Introduction to Biology2h 42m
2. Chemistry3h 37m
3. Water1h 26m
4. Biomolecules2h 23m
5. Cell Components2h 26m
6. The Membrane2h 31m
7. Energy and Metabolism2h 0m
8. Respiration2h 40m
9. Photosynthesis2h 49m
10. Cell Signaling59m
11. Cell Division2h 47m
12. Meiosis2h 0m
13. Mendelian Genetics4h 44m
14. DNA Synthesis2h 27m
15. Gene Expression3h 6m
16. Regulation of Expression3h 31m
17. Viruses37m
- Viruses
  37m
18. Biotechnology2h 58m
19. Genomics17m
- Genomes
  17m
20. Development1h 5m
21. Evolution3h 1m
22. Evolution of Populations3h 53m
23. Speciation1h 37m
24. History of Life on Earth2h 6m
25. Phylogeny2h 31m
26. Prokaryotes4h 59m
27. Protists1h 12m
28. Plants1h 22m
29. Fungi36m
- Fungi
  19m
- Fungi Reproduction
  17m
30. Overview of Animals34m
- Overview of Animals
  34m
31. Invertebrates1h 2m
32. Vertebrates50m
33. Plant Anatomy1h 3m
34. Vascular Plant Transport1h 2m
- Water Potential
  1h 2m
35. Soil37m
- Soil and Nutrients
  20m
- Nitrogen Fixation
  16m
36. Plant Reproduction47m
- Flowers
  33m
- Seeds
  14m
37. Plant Sensation and Response1h 9m
38. Animal Form and Function1h 19m
39. Digestive System1h 10m
- Digestion
  1h 3m
- Blood Sugar Homeostasis
  7m
40. Circulatory System1h 49m
41. Immune System1h 12m
42. Osmoregulation and Excretion50m
- Osmoregulation and Excretion
  50m
43. Endocrine System1h 4m
- Endocrine System
  1h 4m
44. Animal Reproduction1h 2m
- Animal Reproduction
  1h 2m
45. Nervous System1h 55m
- Neurons and Action Potentials
  1h 8m
- Central and Peripheral Nervous System
  46m
46. Sensory Systems46m
- Sensory System
  46m
47. Muscle Systems23m
- Musculoskeletal System
  23m
48. Ecology3h 11m
49. Animal Behavior28m
- Animal Behavior
  28m
50. Population Ecology3h 41m
51. Community Ecology2h 46m
52. Ecosystems2h 36m
53. Conservation Biology24m
- Conservation Biology
  24m

13. Mendelian Genetics

Sex-Linked Inheritance

General Biology

13. Mendelian Genetics

Sex-Linked Inheritance

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0:27 minutes

Problem 12

Textbook Question

Why do more men than women have colorblindness?

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Understand the genetic basis of colorblindness: Colorblindness is primarily caused by mutations in the genes that are responsible for the production of photopigments in the cone cells of the retina. These genes are located on the X chromosome.

Recognize the difference in genetic inheritance between males and females: Males have one X chromosome and one Y chromosome (XY), while females have two X chromosomes (XX).

Consider the impact of having only one X chromosome in males: In males, the presence of a mutation in the only X chromosome will result in colorblindness because there is no second X chromosome to potentially provide a normal copy of the gene.

Examine the inheritance in females: Females must inherit two copies of the mutated gene (one from each parent) to exhibit colorblindness. If they inherit only one mutated gene, the normal gene on the other X chromosome can compensate, typically preventing the development of colorblindness.

Conclude why colorblindness is more common in males: Since males are hemizygous for the X chromosome (having only one X chromosome), any mutation on it expresses the trait of colorblindness. Females, having two X chromosomes, are less likely to be affected as they would require mutations in both copies of the gene.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

X-Linked Inheritance

Colorblindness is primarily caused by mutations in genes located on the X chromosome. Since men have one X and one Y chromosome, a single mutated gene on their X chromosome will result in colorblindness. In contrast, women have two X chromosomes, so a mutation must occur in both to express the condition, making it less common among females.

Genetic Mutation

Colorblindness is often the result of genetic mutations that affect the photopigments in the retina, which are responsible for color perception. These mutations can alter the way light is absorbed by the cones in the eyes, leading to difficulties in distinguishing certain colors, particularly red and green.

Prevalence and Population Genetics

The prevalence of colorblindness varies between genders due to the genetic mechanisms involved. Statistically, about 8% of men and only about 0.5% of women are affected by colorblindness, illustrating how X-linked traits can lead to significant differences in expression between sexes in a population.

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Related Practice

Textbook Question

The pedigree in the figure below illustrates the inheritance of a sex-linked recessive trait. What is the genotype of individual II-5?

a. X^HX^H

b. X^HX^h

c. X^hX^h

d. X^HY

e. X^hY

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Textbook Question

As a genetic counselor, you advise couples about the possibility of genetic disease in their offspring. Today you met with an engaged couple, both of whom are phenotypically normal. The man, however, has a brother who died of Duchenne-type muscular dystrophy, an X-linked recessive condition. His ﬁancée, whose family has no history of the disease, is worried that the couple's sons or daughters might be afflicted. The sister of this man is planning to marry his fiancée's brother. How would you advise this second couple?

A couple are both phenotypically normal, but their son suffers from hemophilia, a sex-linked recessive disorder.

What fraction of their children are likely to suffer from hemophilia?

What fraction are likely to be carriers?

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Textbook Question

Two mothers give birth to sons at the same time in a busy hospital. The son of couple 1 is afflicted with hemophilia A, which is a recessive X-linked disease. Neither parent has the disease. Couple 2 has a normal son even though the father has hemophilia A. The two couples sue the hospital in court, claiming that a careless staff member swapped their babies at birth. You appear in court as an expert witness. What do you tell the jury? Make a diagram that you can submit to the jury.

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Textbook Question

Heather was surprised to discover she suffered from red-green colorblindness. She told her biology professor, who said, 'Your father is colorblind, too, right?' How did her professor know this? Why did her professor not say the same thing to the colorblind males in the class?

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Textbook Question

Recall that hemophilia is an X-linked recessive disease. If a woman with hemophilia had children with a man without hemophilia, what is the chance that their first child will have the disease? What is the chance that their first child will be a carrier?

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Multiple Choice

What is the primary difference between sex-linked traits and autosomal traits in humans?